The field of this invention is the manufacturing of magnetostrictive metal rods. The invention is particularly concerned with the conversion of rare-earth iron alloys into grain-oriented magnetostrictive rods.
In recent years considerable research has been devoted to the development of magnetostrictive compounds, and in particular rare earth-iron alloys. These developments are summarized by A. E. Clark, Chapter 7, pages 531-589, in "Ferromagnetic Materials," Vol. 1, (Ed. E. P. Wohlforth, North-Holland, Publ. Co., 1980). A major objective of the research has been to develop rare earth-iron alloys with large room temperature magnetostriction constants. Technically important alloys having these properties include alloys of terbium together with dysposium and/or holmium. The relative proportions of the rare earths and the iron are varied to maximize room temperature magnetostriction and minimize magnetic anisotropy. Presently, the most technically advanced alloy of this kind is represented by the formula Tb.sub.x Dy.sub.1-x Fe.sub.1.5-2.0 wherein x is a number from 0.27 to 0.35. An optimized ratio is Tb.sub.0.3 Dy.sub.0.7 Fe.sub.1.9 which is known as terfenol-D, as described in U.S. Pat. No. 4,308,474.
Such rare earth-iron alloys are true compounds and can exist in crystalline or polycrystalline form. In preparing elongated bodies (viz. rods) from such alloys, grain-orientation of the crystals is essential for achieving high magnetostriction. An axial grain orientation of the crystallites not only increases the magnetostriction constant but also reduces internal losses at the grain boundaries. This is particularly important in applications where a high magnetostriction at low applied fields is required. (See Clark, cited above, pages 545-547.)
Heretofore the methods employed for preparing grain-oriented rods have not been adapted for large scale commercial manufacture. It is known that grain orientation can be achieved by an induction zoning procedure. See Clark, cited above, page 45; and U.S. Pat. No. 4,308,474, Example 1. As described in the cited patent, partially grain-oriented Tb.sub.0.27 Dy.sub.0.73 Fe.sub.1.98 (terfenol-D) was prepared by horizontal zone procedure. First, the appropriate amounts of the three elements were alloyed by arc-melting into homogeneous buttons of random polycrystalline structure. The buttons were drop cast into rods which were not grain-oriented. The rods were placed in a water-cooled copper tube crucible (cold crucible) extending horizontally in a quartz vacuum chamber. The zone melting operation was carried out under an argon atmosphere with induction heating being used to melt the rods. The melt zone was moved along the horizontal length of the cold crucible.
This forming method had several disadvantages. It required the use of very pure elements, which excluded regular commercial grades of the rare earths terbium, dysprosium and holmium. Lower cost commercial grades of these rare earth metals contain impurities with higher melting points than that of the alloy, such as refractory oxides and carbides. If commercial grade rare earth were employed, the impurities would contaminate the rods and interfere with their desired properties.
Since for optimization the exact ratios of the rare earth metals are critical, it has been necessary in prior procedures to use only small amounts of precisely weighed ingredients and to fuse these into completely homogeneous buttons. There has been a need for a procedure in which complete homogenization of the arc-melted buttons or fingers is not essential, and in which the amounts prepared can be larger.
Another disadvantage of the prior process is that in the zone-melt operation the liquid alloy is in contact with the quartz container for a period of time in which the alloy can be contaminated by the quartz. It would be desirable to avoid the opportunity for quartz contamination of the rods.